1. Field of the Invention
The present disclosure relates to a reflective display device.
2. Description of the Related Art
In recent years, reflective display devices using electrowetting have been gaining attention as a reflective display device for electronic paper.
For example, “Nature” 425, 383-385 (2003) discloses a reflective display device in which colored non-polar liquid (color oil) and transparent and colorless polar liquid (water) are filled within a cell having, on a bottom surface of the cell, a white substrate, transparent electrodes disposed on the white substrate, and an insulating film having a water-repellent front surface and disposed on the transparent electrode. When no voltage is applied, the color oil spreads over an entire surface of the water repellent film and a color of the oil is displayed. When a voltage is applied between the electrodes and the polar liquid, a contact angle of water with the water repellent film is reduced, and a front surface of the water repellent film apparently becomes hydrophilic. This causes a white color of the substrate to be displayed as the water spreads over the bottom surface and the color oil moves to corners. In this manner, displayed colors may he switched by applying a voltage, and high reflectance and contrast may be achieved in a monochrome operation.
Unfortunately, in color display, as it is usually required to use color filters, a loss increases, and, a reflective area for each color decreases in inverse proportion to a number of colors as the different colors are arranged in parallel. Therefore, reflectance and contrast in a polychrome operation are reduced to a large extent as the number of colors increases. In addition, with such a configuration, it is usually difficult to perform a bistable operation required for passive matrix driving.
Unexamined. Japanese Patent Publication No. 2011-65182 discloses a reflective display device having a color shifting pixel configuration and capable of performing a bistable operation. This reflective display device includes an equilateral triangular display surface having triangular sub-pixels of four colors of red (R), green (G), blue (B), and black (K), and K is disposed in center, and R, G, and B are arranged around K. Each sub-pixel is provided with a separate electrode, and an entire pixel is covered by an insulating film having a water repellent surface. The entire pixel is provided with transparent polar water and non-polar black oil. When no voltage is applied, the non-polar black oil spreads over the entire pixel to display black. When a voltage is applied to the R, G, and B sub-pixels, it is possible to achieve display of white (actually, gray). When a voltage is applied to the electrodes at a peripheral portion, R, G, and B are achieved as the colors of the pixel to which the voltage is applied. Further, when a voltage is applied to two of the electrodes excluding one for black, colors of cyan (C), magenta (M), and yellow (Y) are displayed.
Unexamined Japanese Patent Publication No. 2011-65182 also discloses a reflective display device that realizes a bistable operation allowing passive matrix driving. This structure includes a lower substrate, an electrode disposed on the lower substrate, a dielectric layer disposed over the electrode, an upper substrate, a transparent electrode disposed on the upper substrate, and a dielectric layer disposed over the transparent electrode. The upper substrate and the lower substrate are disposed such that the dielectric layers of the both substrates face toward each other, and colored oil and an aqueous layer that is colored by a different color are disposed between the two substrates. When no voltage is applied, the reflective display device displays the color of the aqueous layer. When a voltage is applied between the electrode of the lower substrate and the aqueous layer, the oil turns into small droplets due to charge formation between the aqueous layer and the oil, and makes the dielectric layer of a front surface wet as a driving voltage is cancelled. This achieves a second stable state. Unexamined. Japanese Patent Publication No. 2011-65182 discloses that the electrode disposed on the upper substrate may be disposed on a side wall.
Further, international Publication No. WO2012/039471 discloses a reflective display device capable of performing color display with high contrast. This reflective display device is provided, with a cell configured by a pair of substrates, and layers made of a hydrophilic material are respectively provided for upper and lower surfaces within the cell. Hydrophobic liquid is filled within the cell.
In “Proceedings of the International Symposium on Electronic Paper”, pp 1-6 (2012), a reflective display device capable of performing a bistable operation and having a different structure is disclosed. This reflective display device includes electrodes at a display position as a pixel, and a standby position of droplets, and realizes the bistable operation of moving polar droplets among non-polar droplets by switching the electrodes to which a voltage is applied.